This invention relates to a process for decreasing or eliminating the sulfur content of coal. The presence of sulfur in coal causes many industrial problems. For example, the burning of coal for any purpose, such as the production of electricity, is generally legally limited to coal containing a relatively small amount of sulfur so that the emission of sulfur gases is maintained below specified limits. This is necessary because the production of noxious gases such as SO.sub.2 has an adverse effect on the environment. Many coals contain substantially more sulfur than is permissible for burning. Heretofore, attempts to lower the sulfur content such as by reaction with hydrogen, usually in the presence of a solvent and a hydrogenation catalyst, have not found commercial success, and certain of these processes are destructive of a portion of the coal by conversion thereof to undesired gaseous or liquid products. An alternative method to sulfur removal from coal is to remove noxious gases from the stack gases, and although a great amount of time and money has been spent to develop a suitable process and equipment, only limited commercial success has been obtained. Also, the blending of high sulfur coal with low sulfur coal to bring the sulfur content of the blend to useable limits is not satisfactory in requiring large amounts of very low sulfur content coal.
The presence of nitrogen and oxygen in coal is also undesirable. On burning the coal, the nitrogen is converted to nitrogen oxide gases, or to particulate matter which, while not considered as objectional as the sulfur oxides, are undesirable and their release to the atmosphere regulated. The removal of oxygen from coal upgrades the coal by increasing its heating value, i.e., by increasing the British Thermal Units (BTU) obtained per pound of coal on burning.
An object of the invention is to provide a novel process for decreasing the sulfur content of coal in a facile and economical manner.
A specific object of this invention is to provide a novel process for the substantial removal of sulfur from coal without concomitant conversion of appreciable amounts of coal to other products.
A further object is to provide a novel process for contacting dried coal particles with hydrogen under conditions whereby the hydrogen reacts with the sulfur contained in the coal to produce hydrogen sulfide, thereby reducing the sulfur content of the coal.
A still further object is to provide a novel process for simultaneously decreasing the sulfur, nitrogen and oxygen content of coal.
In an embodiment of the invention, coal particles are dried, mixed with hydrogen at an elevated pressure, and the mixture exposed to wave energy in the microwave range. Under the influence of the microwaves, the hydrogen and sulfur react to form hydrogen sulfide which is removed in the gaseous phase so that the coal particles have a significantly reduced sulfur content. Also, under the influence of the microwaves, the nitrogen and hydrogen may react to form ammonia and the oxygen and hydrogen react to form water, so that the coal particles have a significantly reduced oxygen content and may have a reduced nitrogen content.